(1) Field of the Invention
The present invention relates to the manufacture of semiconductor devices in general, and in particular, to a method of preventing via poisoning and the attendant xe2x80x9cvolcanoxe2x80x9d effect which can cause reliability as well functionality problems especially with sub-micron technologies.
(2) Description of the Related Art
Poisoned vias and the attendant xe2x80x9cvolcanoxe2x80x9d-like eruptions that occur at interconnections between metal layers in semiconductor devices are becoming more and more evident with newer metals, such as copper, and newer dielectrics, such as low-k dielectrics, that are gaining more use in the low sub-micron technologies of to-day. It is disclosed later in the embodiments of the present invention a method of forming interconnects, such as the damascene interconnect, without having the via poison effect.
With the advent of ultra large scale integrated (ULSI) circuit technology, the number of interconnections required between millions of transistors have increased astronomically, as is well known to the practitioners in the art. These interconnections, in the form of metal lines, are usually are of very fine geometries and are closely spaced with respect to each other in order to conserve xe2x80x9creal estatexe2x80x9d in the chip on which they are formed. The planar area of the chip is further conserved by forming multi-level metallized layers separated from each other by insulative layers. The close spacing between the lines, both horizontally on the same layer, and vertically between layers, can cause higher electrical interference and cross-talk between the lines, and hence high resistance-capacitance (RC) delay for the circuitry. As the device geometries shrink further to deep submicron geometries such as 0.15 micrometers (xcexcm), the RC delay becomes even more significant.
In order to decrease the RC delay, or, time constant, within these multi-level integrated systems, low-k (dielectric constant) insulative materials are used. Conventional semiconductor fabrication methods use silicon dioxide or similar insulative materials as both gap filler between adjacent conductor lines on the same layer and as an interlayer insulator between different layers of interconnections. However, low-k materials give rise to via poison effect during the process of manufacturing interconnects, especially copper dual damascene interconnects. The present invention discloses a method of preventing such poison effects in xe2x80x9cviasxe2x80x9d formed between metal layers as well as in xe2x80x9ccontactsxe2x80x9d that are formed between the devices in the semiconductor substrate and the first level metal, as is known in the art.
Cu dual damascene is preferred as an interconnect because, as is well known in the art, copper has lower resistivity than the commonly used aluminum and, therefore contributes to lower RC delay. The damascene process also provides a better control of the metal line geometries, as described below, and therefore improves further the RC characteristics of the lines. However, if the damascene structure is not properly protected during forming of the contact and via holes, the holes can be xe2x80x9cpoisonedxe2x80x9d due to outgassing from the insulative layers, and/or due to the hydrophobic nature of the insulative layers. A poisoned contact hole (reaching the substrate), or a poisoned via hole (connecting different metallized layers) can give rise to voids, cavities for contaminants to enter, poor interfaces between contacting conductors, and, hence, poor connections between interconnects. It is disclosed later in the embodiments of the present invention a method of protecting dual damascene structures in order to avoid via poisoning problem and the attendant poison effect.
In one approach for a dual damascene process shown in FIG. 1a, two insulative layers (120) and (130) are formed on a substrate (100) with an intervening etch-stop layer (125). Substrate (100) is provided with metal layer (110) and a barrier layer (115). Metal layer can be the commonly used aluminum or copper, while the barrier can be an oxide layer. A desired trench or trench pattern (150) is first etched into the upper insulative material (130) using conventional photolithographic methods and photoresist (140). The etching stops on etch-stop layer (125). Next, a second photoresist layer (160) is formed over the substrate, thus filling the trench opening (150), and patterned with hole opening (170), as shown in FIG. 1b. The hole pattern is then etched into the lower insulative layer (120) as shown in FIG. 1c and photoresist removed, thus forming the dual damascene structure shown in FIG. 1f. 
Or, the order in which the trench and the hole are formed can be reversed. Thus, the upper insulative layer (130) is first etched, or patterned, with hole (170), as shown in FIG. 1d. The hole pattern is also formed into etch-stop layer (125). Then, the upper layer is etched to form trench (150) while at the same time the etching transfers the hole pattern in the etch-stop layer into lower insulation layer (120), as shown in FIG. 1e. It will be noted that the etch-stop layer stops the etching of the trench into the lower insulation layer. After the completion of the thusly formed dual damascene structure, both the hole opening and trench opening are filled with metal (180), and any excess material on the surface of the substrate is removed by chemical mechanical polishing, as seen in FIG. 1f. 
However, when trench (150), or hole (170) openings are formed through the insulative layers (120) and (130) as shown in FIGS. 1b-1e, moisture (190) is absorbed from the atmosphere by the exposed dielectric layers on the sidewalls of the openings. After copper (180) is deposited, moisture (190) is then released from the dielectric layers. This moisture diffuses into the metal causing poisoned via/contact metallurgy.
Other forms of via poisoning and the attendant volcano effect can also occur during the processing of vias and contacts in semiconductor manufacturing, especially when interconnect holes are filled with photoresist in an attempt to protect the holes from damage during subsequent process steps. A dual damascene with a sacrificial fill is described in U.S. Pat. No. 5,705,430 by Avanzino, et al. A first layer of insulating material is formed with via openings. The openings arc filled with a sacrificial removable material. A second layer of insulating material is deposed on the first layer. In one embodiment, the etch selectivity to the etchant of the second layer is essentially the same as the sacrificial via fill and, preferably, is substantially higher than second layer. Using a conductive line pattern aligned with the via openings conductive line openings are etched in the second insulating layer and, during etching, the sacrificial fill is removed from the via openings. In a second embodiment. The sacrificial material is not etchable by the etchant for forming the conductive line openings and, after formation of the conductive line openings, the sacrificial material is removed with an etchant to which the first insulating layer is resistive or less selective. A conductive material now is deposited in die conductive line and via openings. If, however, the sacrificial fill is a photoresist, where the photoresist can be removed by any number of methods including oxygen plasma ashing, then the residues found in the via holes can cause volcano-like eruptions in later process steps. Another method of forming a dual damascene structure using a sacrificial stud in the via hole is shown in U.S. Pat. No. 6,033,977 by Gutsche, et al. However, in this case, the sacrificial material is selected from the group consisting of flowable oxide, CVD oxide and boron silicate glass.
Another method for producing a metallization level having contact and interconnect connecting the contacts is taught by Zettler, et al., in U.S. Pat. No. 5,422,309. An insulating layer wherein contact holes to regions to be contacted are opened is applied surface-wide onto a substrate. For producing an interconnect mask, a photoresist layer is applied, exposed and developed such that the surface of the regions to be contacted remains covered with photoresist in exposed regions, whereas the surface of the insulating layer is uncovered in the exposed regions. Using the interconnect mask as etching mask, trenches are etched into the insulating layer. Contacts and interconnects of a metallization level are finished by filling the contact holes and the trenches with metal.
Azuma in U.S. Pat. No. 6,051,369 discloses a method of forming a dual damascene using one or more antireflective coating films. A film is formed on the antireflective film and a radiation sensitive film is formed on the film. The radiation sensitive film is selectively exposed. During the selective exposing, the antireflective film covers the lower surface of the portion of film on which the radiation sensitive film is formed, and the antireflective coating film reduces reflections of radiation during the selective exposing of the radiation sensitive film. A fabrication process using the lithography process is also described.
In still another U.S. Pat. No. 5,741,626, Jain, et al., cite the forming of a dual damascene using a particular etch process. The method provides an anti-reflective Ta3N5 coating which is used in a dual damascene structure and for I line or G line lithographies. In addition, the Ta3N5 coating may also used as an etch stop and a barrier layer. A dual damascene structure is formed by depositing a first dielectric layer. A dielectric tantalum nitride layer is deposited on top of the first dielectric layer. A second dielectric layer is deposited on the tantalum nitride layer. A dual damascene opening is etched into the dielectric layers by patterning a first opening portion and a second opening portion using photolithography operations. Dielectric tantalum nitride layer serves as an ARC layer during these operations to reduce the amount of reflectance from conductive region to reduce distortion of the photoresist pattern. The use of a dielectric tantalum nitride layer as an ARC is shown particularly for i-line and g-line lithography.
As i-line or g-line lithographies are getting to be more prevalent in applications such as dual damascene, there is a need for improved methods to protect contact/via holes from experiencing poison effects.
It is therefore an object of this invention to provide a method of eliminating poison effect in forming contact/via hole openings in dual damascene structures.
It is another object of the present invention to provide a method of eliminating poison effect by filling contact/via holes with i-line photoresist (i-line PR) which does not leave a scum-like residue that would, otherwise, erupt like a volcano in later process steps.
It is yet another object of the present invention to provide a method of reducing the RC delay of Cu dual damascene interconnects through prevention of poison effect in the damascene structure.
These objects are accomplished by providing a substrate having a passivation layer formed over a first metal layer formed on said substrate; forming a first insulative layer over said substrate; forming an optional etch-stop layer over said first insulative layer; forming a second insulative layer over said etch-stop layer; forming a first photoresist layer over said second insulative layer and patterning said photoresist to form a first photoresist mask having a hole pattern; etching said first and second insulative layers, including said optional etch-stop layer through said hole pattern to form a hole reaching said passivation layer; removing said first photoresist mask; forming a fill material over said substrate, including in said hole opening; removing any excess fill material over said hole opening; forming a second photoresist layer over said substrate, including said hole opening and patterning said second photoresist to form a second photoresist mask having a trench pattern; etching said second insulative layer through said trench pattern in said second photoresist mask to form a trench in said second insulative layer, thus completing the forming of said dual damascene structure in said substrate; removing said second photoresist mask; removing said fill material from said hole opening; depositing a second metal in said dual damascene structure; and removing excess metal to complete the forming of said dual damascene without the poison effect. These objects are further accomplished by using as fill material for this application, i-line photoresist (PR), or spin-on organic oxides, such as SiLK or FLARE.